1. Field of the Invention
The present invention relates to an automotive radar system, and more particularly to an automotive radar system suitable for use in systems for Adaptive Cruise Control (ACC), Forward Collision Warning System(FCWS), and Automotive Braking for Reduction of Collision Speed.
2. Description of the Related Art
As disclosed in Japanese Publication of Unexamined Patent Application No. 11-39586, for example, systems for Adaptive Cruise Control (ACC) and Forward Collision Warning (FCW) are intended to not only perform follow-up running while automatically adjusting a vehicle speed so that the vehicle distance between a preset target and a radar-loaded vehicle, but also to issue a warning.
Also, an automatic lane change system to avoid a collision against an obstacle is intended to keep a harmonic balance between automatic control and steering operation of the driver as disclosed in Japanese Publication of Unexamined Patent Application No. 2000-142281, for example.
Further, as disclosed in Japanese Publication of Unexamined Patent Application No. 2001-80491, such an automatic lane change system is implemented as a steering force control system for increasing a gain in yawing moment control to increase the yawing moment depending on a steering angular speed and a steering angular acceleration in the case of urgent lane change.
A radar is employed as a range sensor in the automatic lane change system to avoid a collision and the ACC system described above. As a radar, there is generally employed a laser radar or a millimeter wave radar.
A radar emits a radio beam or a laser light, which is reflected by an object, such as a vehicle or an obstacle, and measures a time required for the beam until coming back to the radar. Based on the measured time, the radar determines the distance up to the object having reflected the beam.
Of the two types of radars, the laser radar using the laser light emits a thin laser beam. To receive the laser light with stable intensity, therefore, it is desired that the target have a reflecting surface serving as a light reflector.
On the other hand, of the two types of radars, the millimeter wave radar using a millimeter wave is able to capture the target with higher stability even in a state of rain or fog as compared with the laser radar, and therefore it is expected as an all-weather sensor.
Regarding the millimeter wave radar, there are known several methods for measuring the distance up to the object and the relative velocity relative to the object.
For example, xe2x80x9cDevelopment Trend of Automotive Millimeter Wave Radarsxe2x80x9d, The Institute of Electronics, Information and Communication Engineers, October 1996, pp. 977-981, describes various measuring methods including a 2-Frequency CW (Continuous Wave) method switching over two frequencies from one to the other and an FMCW (Frequency Modulated Continuous Wave) method carrying out triangular modulation on transmission frequency.
In those 2-Frequency CW method and FMCW method, a received signal is subjected to the FFT (Fast Fourier Transform) process, and the distance up to the object and the relative velocity relative to the object are measured from information regarding the frequency, the phase and the amplitude of a peak signal in a frequency spectrum obtained by the FFT process.
The above-described millimeter wave radar of the related art has various superior advantages, such as being of all-weather type, to the laser radar, but it accompanies with problems given below.
The radio beam emitted from an onboard millimeter wave radar is not a narrow beam unlike a laser beam, and propagates with a certain spread. When radiating the radio beam to a target having a size comparable to that of a passenger automobile, therefore, even if the passenger automobile deviates from the center position of the radio radiation during running because of changes of the radio radiation angle, the radio beam is regarded as being radiated to the passenger automobile. Therefore, a variation is increased in detection of the crossrange, i.e., the distance from an extension of a center axis of the radar-loaded vehicle to the passenger automobile as the target.
To suppress a variation in detection of the crossrange and to improve detection accuracy, a gain of a tracker filter used in a radar output estimating section is usually set to a relatively small value.
However, when changing the lane to avoid a collision against an obstacle, the yaw angle and the crossrange of the radar-loaded vehicle are changed to a large extent. This causes a large phase delay in the radar output at the small gain of the tracker filter, which is usually set.
In the onboard millimeter wave radar, as described above, a millimeter wave is radiated with a certain spread. Accordingly, when the radar-loaded vehicle approaches the target vehicle in a condition in which the crossrange is offset, there occurs a phenomenon that the power intensity of radio waves reflected by a side surface of the target vehicle continues to increase and the center position of the reflected radio waves moves from the center position of the vehicle width toward the vehicle side surface.
For that reason, when a driver changes the lane to avoid a collision against an obstacle, a crossrange response of the detected obstacle causes a phase delay as compared with the behavior of lane change of the radar-loaded vehicle, thus resulting in a phenomenon of detection delay.
Accordingly, it is an object of the present invention to provide a radar system, which can increase a crossrange detection speed in the lane change state by employing responses of sensors loaded on a vehicle, such as a steering angle sensor and a gyro sensor, without providing additional hardware.
To achieve the above object, the present invention is constructed as follows.
(1) In an automotive radar system for detecting at least one of a range, a crossrange, an azimuth and a relative velocity relative to a target, the system comprises a lane-change behavior state detecting unit for detecting whether a radar-loaded vehicle is in a lane-change behavior state; and a detection response speed changing unit for changing a target detection response speed of the radar system to a value larger than that in a running state other than the lane-change behavior state when the lane-change behavior state detecting unit detects that the radar-loaded vehicle is in the lane-change behavior state.
(2) In the automotive radar system set forth in above (1), preferably, the detection response speed changing unit comprises a filtering unit for executing a filtering process with a smoothing effect on at least one of the range, the crossrange, the azimuth and the relative velocity relative to the target, and a filter gain changing unit for increasing a gain of the filtering unit, wherein the target detection response speed is set to a value larger than that set in the running state other than the lane-change behavior state by increasing the gain of the filtering unit.
(3) In above (1), preferably, the lane-change behavior state detecting unit includes at least one of a yaw rate or lateral acceleration sensor, a steering angle sensor, a steering torque sensor, a yaw rate sensor, a tire pressure sensor, and a lateral acceleration sensor, and detects the lane-change behavior state of the radar-loaded vehicle in accordance with an output of the at least one sensor.
(4) In above (1), preferably, the lane-change behavior state detecting unit includes a radar-loaded-vehicle lane determining unit for calculating a status variable given as a second time-derivative status variable representing a change amount of a steering angular speed or a change amount of a yaw rate, and then determining that the radar-loaded vehicle is in the lane-change behavior state, when the calculated status variable exceeds a predetermined level.
(5) In above (1), preferably, the automotive radar system further comprises a radar detected value modifying unit for modifying at least one of the range, the crossrange, the azimuth and the relative velocity relative to the target by using a longitudinal or transverse momentum and/or a speed of the radar-loaded vehicle when the lane-change behavior state detecting unit detects that the radar-loaded vehicle is in the lane-change behavior state.
(6) In above (1), preferably, the automotive radar system further comprises a radar-loaded-vehicle transverse movement detecting unit for detecting a longitudinal momentum, a transverse momentum and/or a speed of the radar-loaded vehicle by using an output of a vehicle speed sensor, an acceleration sensor and/or a steering angle sensor.
(7) In above (1), preferably, a forward collision warning is issued when the crossrange from a center of the radar-loaded vehicle to the target is detected as being less than a predetermined value, and the forward collision warning is automatically stopped when the crossrange from the center of the radar-loaded vehicle to the target is detected as being not less than the predetermined value.
(8) In above (1), preferably, deceleration control with braking is performed when the crossrange from a center of the radar-loaded vehicle to the target is detected as being less than a predetermined value, and the deceleration control with braking is automatically stopped when the crossrange from the center of the radar-loaded vehicle to the target is detected as being not less than the predetermined value.
(9) Also, in an automotive radar system for detecting at least one of a range, a crossrange, an azimuth and a relative velocity relative to a target, the system comprises a lane-change behavior state detecting unit for detecting whether a radar-loaded vehicle is in a lane-change behavior state; and a radar detected value modifying unit for modifying at least one of the range, the crossrange, the azimuth and the relative velocity relative to the target by using a longitudinal and/or transverse momentum or a speed of the radar-loaded vehicle when the lane-change behavior state detecting unit detects that the radar-loaded vehicle is in the lane-change behavior state.
(10) Further, in an automotive radar system for detecting at least one of a range, a crossrange, an azimuth and a relative velocity relative to a target, the system comprises a lane-change behavior state detecting unit for detecting whether a radar-loaded vehicle is in a lane-change behavior state; and a warning stopping unit for issuing a forward collision warning when the crossrange from a center of the radar-loaded vehicle to the target is detected as being less than a predetermined value, and automatically stopping the forward collision warning when the crossrange from the center of the radar-loaded vehicle to the target is detected as being not less than the predetermined value.
(11) Still further, in an automotive radar system for detecting at least one of a range, a crossrange, an azimuth and a relative velocity relative to a target, the system comprises a lane-change behavior state detecting unit for detecting whether a radar-loaded vehicle is in a lane-change behavior state; and a brake deceleration canceling unit for performing deceleration control with braking when the crossrange from a center of the radar-loaded vehicle to the target is detected as being less than a predetermined value, and automatically stopping the deceleration control with braking when the crossrange from the center of the radar-loaded vehicle to the target is detected as being not less than the predetermined value.
When the radar-loaded vehicle changes its lane, a response in detection of the crossrange relative to a forward vehicle is maximized with more importance attached to the detection response than to a variation in the crossrange detection.
Also, the lane change state of the radar-loaded vehicle is determined using the steering angle sensor, the angular speed sensor, a turn signal lamp, etc., which are existing ones.
As a result, a radar system can be realized, which has a faster crossrange detection speed in the lane change state by employing responses of sensors loaded on a vehicle, such as a steering angle sensor and a gyro sensor, without providing additional hardware.